MRT devices have long been used in medicine and biophysics to obtain images from inside the body of an examination object (a patient for example). Magnetic resonance tomography is based on the physical phenomenon of nuclear spin resonance. With these examination methods, the examination object is exposed to a strong constant basic magnetic field by means of which the previously motionless orientated nuclear spin of atoms is aligned. This extraordinarily strong basic magnetic field is generated by a superconductive basic field magnet and typically amounts to 0.22 to 1.5 Tesla. These unordered nuclear spins are activated to a specific oscillation (resonant frequency) by means of high frequency waves (HF signals) emitted via HF antennas into the examination area. In resonance tomography, this oscillation generates the actual measurement signal (HF response signal and/or nuclear resonance signal) for the MRT imaging, which is recorded using suitable receiving coils (once again typically via HF antennas).
Accurate information (local information and/or local coding) about the relevant point of origin of the HF response signal in the examination object is needed to attain an image. This local information is attained by means of magnetic option fields (magnetic gradient fields) which are generated along the three spatial directions of gradient coils (two saddle coils arranged orthogonally to each other and a Maxwell coil). The gradient fields overlaying the basic magnetic field are designed such that the magnetic field strength and thus the resonant frequency are different in each volume element. If a defined resonant frequency is emitted across the HF antenna, only atoms which are found at a location in the examination area are activated, at said location the magnetic field and the overlay of the basic field with the gradient fields achieving the corresponding resonance condition. Suitable variations of the gradient fields enable a defined movement of the location of a volume element, whereby the resonance condition is achieved, so that the area of interest can be scanned voxel-by-voxel.
Each of these gradient coils is equipped with its own power supply in order to accurately generate independent current pulses according to consequential amplitudes and time programmed in a pulse sequence control of a system computer. The currents required lie at approximately 250 A. Slew rates with an order of magnitude of 250 kA/s are necessary as the gradient circuit times should be as short as possible. Strong mechanical oscillations are related to this type of switching in the extraordinarily strong basic field as a result of the resulting Lorentz force, said oscillations making correspondingly high mechanical demands on the whole system. Furthermore, the high current strengths cause high ohmic losses which result in a powerful heating of the gradient coils. As a result of this, active cooling is necessary, (a water heat exchanger for example), the functions of which are continuously monitored by means of thermosensors and/or coil monitoring lines.
For the purpose of a nuclear tomographic examination, the object (generally the patient) is brought into the examination room of the MRT device and positioned. In order to enable the recording of examination objects of different parameters, the examination room must be of a minimum size. With medical applications, the size is selected such that a patient can be completely transported into the examination room.
For the examination of a special area of the examination object, the head of a patient for example, a so-called local gradient coil unit (subsequently referred to as ‘insert gradient coil’) can be used. U.S. Pat. No. 5,185,576 discloses this type of insert gradient coil unit, which is brought into the examination room of the MRT device.
FIG. 1 shows a schematic section through an MRT device with integrated insert gradient coils according to the prior art. The section shows a large shaded area 1 which comprises the superconductive basic field magnets and the whole body gradient coil system of the HF antenna. For design reasons, the components (not shown individually) mentioned previously are surrounded by a suspension tube 2 which is enlarged in the trumpet shape at both front sides 3, 4, and surrounds and restricts an internal chamber. Guide rails 7 are located in the lower region of the internal chamber, on said guide rails a patient support 6 with a head support 8 is inserted into the internal chamber 5 by means of the front 3 (right side in this case) of the suspension tube end. An insert gradient coil 9 is located in the middle area of the internal chamber 5, said insert gradient coil being designed for examinations in the head region of a patient and in which the head support 8 is accommodated for this purpose.